The Answer Is...

You can detect the answer in Pope Gregory's instructions to St. Augustine regarding the conversion of the pagan peoples to Christianity:

"Destroy the idol. Purify the temples with holy water. Set relics there, 
and let them become temples of the true God.  So the people will have no
need to change their place of concourse, and, where of old they were wont 
to sacrifice cattle to demons, thither let them continue to resort on the 
day of the sint to where the Church is dedicated, and slay their beasts,
no longer as a sacrifice but for social meal in honor of Him whom they
now worship."

In other words, to facilitate conversion to Christianity, pre-Christian practices and symbols were incorporated into the rituals of the Catholic Church.

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